This invention is directed to the strengthening of transparent glass-ceramic articles which exhibit low linear coefficients of thermal expansion. Glass-ceramic bodies conventionally exhibit greater mechanical strengths than the precursor glasses from which they are crystallized in situ. To illustrate, flexural strengths measured on abraded canes of standard annealed commercial glasses tend to range between about 4000-5000 psi. By comparison, flexural strengths measured on abraded canes of opaque glass-ceramics average about 12,000 psi. This significant improvement in strength permits the fabrication of articles which will be exposed to physical abuse having thinner walls than similar articles prepared from glass That capability has the advantage of enabling the production of articles of lighter weight, but with equivalent or even greater strength For example, a PYREX.RTM. baking dish prepared from a borosilicate glass has much thicker walls and, hence, weighs considerably more than the same dish fashioned from CORNING WARE.RTM. glass-ceramic, both products being marketed by Corning Incorporated, Corning, N.Y.
In contrast to opaque glass-ceramic materials, highly transparent glass-ceramics have conventionally demonstrated abraded flexural strengths averaging only about 8000 psi. Such relatively low mechanical strengths preclude the use of those materials in thin walled articles which may be exposed to physical abuse. Moreover, the potential for enhancing the strength of transparent glass-ceramic bodies is severely limited due to the very small size of the crystals present therein (commonly less than 0.1 micron) and the very low linear coefficients of expansion demonstrated by the bodies. Thus, classic techniques of improving strength, e.g., by some form of dispersion strengthening, would also impair transparency. The probable loss of transparency also rules out other standard techniques such as increasing the extent of crack-microstructure interaction through grain size coarsening. In addition, the very low coefficient of thermal expansion eliminates to a large extent the use of thermal tempering, in view of the fact that the bodies have a very low capacity to absorb residual strain energy during any quenching.
The technique of dilatation strengthening for use with glass-ceramics exhibiting low coefficients of thermal expansion was first described in U.S. Pat. Nos. 4,391,914 (Beall). As explained there, the technique is applied to glass-ceramics containing a lithium aluminosilicate (commonly a .beta.-quartz solid solution and/or a .beta.-spodumene solid solution) as the predominant, if not sole, crystal phase which exhibits a very low linear coefficient of thermal expansion, and a substantial amount of a residual glassy phase (normally a borosilicate or an aluminoborosilicate) which displays a much higher coefficient of thermal expansion. The two phases form a dilatant system wherein the thermal expansion curve changes markedly in character at a transition point in the range of about 500.degree.-750.degree. C.; the crystal phase dominating the expansion below that range and the glass phase controlling at the higher temperatures. The presence of .beta.-quartz solid solution resulted in a body exhibiting high transparency, whereas the occurrence of .beta.-spodumene solid solution resulted in translucent-to-opaque bodies.
As is disclosed in that patent, the residual glass phase comprises about 15-30% by volume of the crystallized body with the overall composition of the glass-ceramic consisting essentially, expressed in terms of weight percent on the oxide basis, of 2.5-7% Li.sub.2 O, 2-5% B.sub.2 O.sub.3, 14-25% Al.sub.2 O.sub.3, 0-2% MgO and/or ZnO, 60-80% SiO.sub.2, 3-6% TiO.sub.2 and/or ZrO.sub.2, and 0.5-5% mole percent of glass modifying oxides selected from the group consisting of Na.sub.2 O, K.sub.2 O, CaO, BaO, SrO, and PbO, wherein the molar ratio Al.sub.2 O.sub.3 :Li.sub.2 O&gt;1. In the preferred embodiment the linear coefficient of thermal expansion of the glass phase will be at least 30.times.10.sup.-7 /.degree.C. units greater than that of the crystal phase.
The patent cited two mechanisms underlying the enhancement in strength possible through the dilatation strengthening technique. Hence, because the glass phase has a significantly higher coefficient of thermal expansion than the crystal phase, it expands faster as the body is heated. Conversely, the glass contracts faster than the crystal when the body is cooled. The volume of glass (.apprxeq.15-30%) is such as to form a continuous matrix at the temperatures employed to crystallize the precursor glass body in situ into a glass-ceramic; the crystals, while dominant in amount, are nevertheless separated in the glass matrix. As the crystallized body cools, however, the higher expansion glass contracts more rapidly and a point is reached where the crystals touch one another to form a crystalline network with the continuous glass network being destroyed and the glass occupying interstitial positions. This contraction of the glass sets up point compressive stresses in the crystalline network. As a consequence, fracture impediment due to a transgranular pattern develops with resultant fracture toughness.
More important to the development of increased strength, however, is the shape of the thermal expansion curve and its applicability to thermal tempering. The composition of the material is tailored so that the glassy phase is barely continuous, whereby its shrinkage causes geometric isolation at some temperature during cooling. On further cooling, the crystalline network controls the contraction curve. Therefore, when the surface of the glass-ceramic is quenched from the crystallization temperature, the body shrinks and quickly arrives at a rigid state of a continuous crystalline network. The body will not shrink further as the body cools still further, providing that the coefficient of thermal expansion of the crystals is close to zero. The interior of the body is cooling much slower, however, and must contract through the glass dominated, high thermal expansion regime while the body surface is rigid and not contracting. Stress in the form of surface compression is thereby induced, the magnitude thereof depending upon the volume shrinkage of the upper part of the expansion curve. Inasmuch as the glassy phase is normally somewhat plastic in the high temperature region, it withstands the severe shock of surface quenching. When the surface becomes entirely rigid, it exhibits a sufficiently low thermal expansion (being crystal dominated) to avoid cracking on further rapid cooling to room temperature.
For additional discussion regarding the mechanisms underlying dilatation strengthening, reference is made to Pat. No. 4,391,914, which patent is incorporated in full in the present disclosure.
Whereas abraded modulus of rupture values of 25,000 psi and even greater were measured on the opaque, betaspodumene solid solution-containing bodies and up to 20,000 psi on the statedly transparent beta-quartz solid solution-containing bodies described in U.S. Pat. No. 4,391,914, the dilatation strengthening practice to date has not been commercially realized with transparent articles. Thus, the practice has led to the production of haze, i.e., a loss of transparency, even in the clearest of the .beta.-quartz solid solution-containing articles. And generally, those compositions exhibiting the highest tempered strengths also manifested the highest degree of haze. Furthermore, the time and the temperature range wherein a particular precursor composition could be crystallized in situ to a product exhibiting a minimum level of haze was very limited, thus demanding extreme care and control in carrying out the crystallization process.
Accordingly, the principal objective of the present invention was to develop high strength, i.e., abraded modulus of rupture values in the vicinity 10,000 psi, transparent glass-ceramic bodies which are essentially free from haze, wherein the predominant and substantially sole crystal phase is .beta.-quartz solid solution which exhibits a linear coefficient of thermal expansion of less than 10.times.10.sup.-7 /.degree.C.
A second and related objective was to discover compositions for such products which can be crystallized in situ over a relatively broad range of temperatures.